A pressure-sensitive adhesive is widely used for adhesion to the surfaces of various adherends such as metals, glasses, woods, papers, corrugated boards, plastic materials, etc. Specific examples of a pressure-sensitive adhesive include acrylic pressure-sensitive adhesives each made up of a polymer mainly comprising an acrylic monomer such as butyl acrylate, 2-ethylhexyl acrylate, etc., as the base polymer, and rubber-based pressure-sensitive adhesives each made up of rubber-based polymer such as a natural rubber, a styrene-isoprene-styrene block polymer, etc., as the base polymer.
In these pressure-sensitive adhesives, to increase the adhesive strength to a specific adherend, it has been performed to introduce therein a functional group such as a carboxyl group, an amino group, etc. For example, in the acrylic pressure-sensitive adhesive, a functional group- containing monomer copolymerizable with a main monomer such as acrylic acid, dimethylaminoethyl methacrylate, etc., is copolymerized and in the rubber-based pressure-sensitive adhesive, an additive such as a tackifier resin having a functional group is used or a functional monomer is grafted onto the rubber molecular chain.
Contrary to this, to weaken the adhesive strength of a pressure-sensitive adhesive to an adherend to impart a releasing property to the adhesive, a monomer having a non-adhesive functional group such as a long-chain alkyl group is copolymerized with or grafted to the adhesive, or an additive having a non-adhesive functional group is compounded with the adhesive. Further, as a modification method of functional group on a pressure-sensitive adhesive surface for increasing or weakening the adhesive strength to an adherend, a method of coating a diluted solution of a function group-containing compound on the surface of a pressure-sensitive adhesive is known.
However, the above-described conventional techniques have the fault that the modification of functional group on the pressure-sensitive adhesive surface is hard to carry out sufficiently. This is because even when the functional group is introduced into the phase of the pressure-sensitive adhesive, it frequently happens that because of the insufficient concentration of the function group at the surface of the pressure-sensitive adhesive, the sufficient effect cannot be obtained, or the molecular chain segment containing the functional group causes a phase separation, whereby the functional group does not uniformly exist at the surface of the pressure-sensitive adhesive. Also, particularly, in the case of a polar functional group such as an amino group and a carboxyl group, the functional group is hard to exist at the surface of the pressure-sensitive adhesive and there is a tendency that the functional group diffused in the inside of the pressure-sensitive adhesive with the passage of time to lower the concentration of the functional group at the surface of the pressure-sensitive adhesive.
When a too much amount of the functional group is introduced in to the pressure-sensitive adhesive to overcome these faults and increase the concentration of the functional group at the surface of the pressure-sensitive adhesive, there is a possibility of hindering the flow characteristics and the adhesive characteristics of the pressure-sensitive adhesive. Also, in such a case, a problem of staining the surface of an adherend, to which the pressure-sensitive adhesive is adhered, with a functional group-containing additive bled out onto the surface of the pressure-sensitive adhesive is liable to occur.
Also, in the method of coating a diluted solution of a functional group-containing compound on the surface of the pressure-sensitive adhesive, by evaporating off the solvent after coating, a thin layer of the functional group-containing compound is formed on the surface of the pressure-sensitive adhesive but in this case, the functional group-containing compound is repelled to form uneven coating and the solvent dissolving the functional group-containing compound dissolves or swells the surface of the pressure-sensitive adhesive, whereby the thin layer of the functional group-containing compound sometimes cannot be formed well.
Furthermore, according to the above-described coating method of a diluted solution, the fixed strength of the functional group-containing compound is liable to become insufficient, whereby the thin layer of the functional group-containing compound is liable to be damaged by friction, etc. In addition, as a general functional group-modificaton method in other fields than the pressure-sensitive adhesive field, a modificaton method of functional group on surfaces used for modifying the surfaces of plastic films, such as a corona discharging treatment, a plasma treatment, a radiation treatment, a photo-graft polymerization, etc., are known, the methods of this kind are not so effective for a soft composition such as a pressure-sensitive adhesive.
An object of the present invention is to overcome these problems in the conventional techniques and to provide a modification method of functional group on a pressure-sensitive adhesive surface capable of safely introducing at high density functional group(s) effective for the improvement and the control of a pressure-sensitive adhesive property to the surface of a pressure-sensitive adhesive.